Well...if they use the internet to shop online a mobile app should not be that hard to deal with given it's installed/configured by the bank clerk. All the mobile phones are "smart phones" now anyway.
If she can't handle a smartphone, can she handle internet purchases? If not, the whole discussion is moot because the topic at hand is online payments and their MFA.
Plenty of dumb phones available at the shopping malls over here.
Besides the UX of the Internet is not the same as the phone and these are the kind of users that end up with the browser full of extensions trying to make pesky dialogues go away.
My grandma can't even use a smart phone to call people with. She only answers if you call because she is literally terrified of technology.